The Global System for Mobile Communication (GSM) cellular system has recently begun service using a new modulation standard referred to as Enhanced Data rates for GSM Evolution (EDGE). This standard uses 8-Level Phase Shift Keying (8PSK) modulation. Transmitters operating according to the EDGE standard must include a modulator providing both amplitude modulation and phase modulation. To efficiently amplify such modulation, a polar system is desirable.
Polar transmitters may be classified as either open loop or closed loop. In an open loop polar transmitter, a transmit signal is broken into amplitude and phase components. The phase component is provided to the radio frequency (RF) input of a power amplifier. The amplitude component may be used to vary the supply voltage, or collector voltage, of the power amplifier such that the output power of the power amplifier follows the instantaneous amplitude of the modulation envelope, thereby providing amplitude modulation.
One issue for an open loop polar transmitter is that amplitude modulation to phase modulation (AM/PM) distortion of the power amplifier varies as the supply voltage varies. One solution to this problem is to pre-distort the phase component prior to amplification in order to compensate for the AM/PM distortion of the power amplifier. However, if the Voltage Standing Wave Ratio (VSWR) at the output of the power amplifier changes due to variations in load impedance, then the AM/PM distortion also changes. As a result, the pre-distortion no longer aligns with the AM/PM distortion of the power amplifier. At low output power levels, this is generally not an issue because the spectrum and Error Vector Magnitude (EVM) requirements of the EDGE standard are less stringent for low output power levels. At higher output power levels, this may become more of an issue because the spectrum and EVM requirements are more stringent.
A closed loop polar transmitter may be used to solve the issue of AM/PM distortion by enclosing the power amplifier within a phase-locked loop (PLL) generating the phase modulation. However, one issue for a closed loop polar transmitter is that strong interference signals present at the antenna are fed back to the PLL along with the output of the power amplifier. If the interference signal is strong enough, the interference signal will overpower the feedback from the output of the power amplifier and cause the PLL to unlock. As a result, the output of the polar transmitter is severely distorted.
Commonly owned and assigned U.S. patent application Ser. No. 11/070,704, entitled CLOSED LOOP POLAR MODULATION SYSTEM WITH OPEN LOOP OPTION AT LOW POWER LEVELS, filed Mar. 2, 2005, currently pending, which is hereby incorporated herein by reference in its entirety, discloses a polar transmitter that is configurable as either an open loop polar transmitter or a closed loop polar transmitter. The polar transmitter may be configured as an open loop polar transmitter for output power levels less than a predetermined threshold and as a closed loop polar transmitter for output levels greater than the predetermined threshold. Switching between the open loop configuration and the closed loop configuration is accomplished by switching a feedback path of a phase-locked loop (PLL) between an output of a power amplifier in the transmit chain when operating as a closed loop polar transmitter to an output of the PLL when operating as an open loop polar transmitter. However, the phase difference between the output of the power amplifier and the output of the PLL may be any angle from 0 to 360 degrees. This phase difference may cause the output of the PLL to deviate from the desired frequency such that the transmitter violates specification.
For example, for a polar transmitter performing EDGE modulation, the European Telecommunications Standardization Institute (ETSI) switching spectrum specification is −23 dBm in a 30 kHz resolution bandwidth at an offset frequency of 400 kHz. The switching spectrum is measured on a peak basis. Thus, if a transmitter fails the −23 dBm limit for any instant in time, the transmitter is out of specification. If the output power of the transmitter is −10 dBm to −5 dBm at the beginning of ramp-up for a transmit burst and the transmitter is switched from open loop to closed loop, the PLL may be momentarily pulled off of the desired frequency by 400 kHz due to the phase difference between the output of the power amplifier and the output of the PLL. As a result, the peak power level at 400 kHz may be exceeded, thereby violating the ETSI switching spectrum specification.
Thus, there remains a need for a system and method for switching from one PLL feedback source to another with minimal phase disturbance.